I was watching videos on the Su-27 again, and this fighter appears to be a heavy version of jet fighters, which means it has longer range and can carry more weight.

This got me thinking on what is the upper limits of fighters, in terms of size and weight. Is there a fundamental limitation on how big a jet can be and still pull 9+ g's and use very high angle-of-attacks?

Why don't they make them any bigger to get more range (or endurance)? Is there a practical limit? Would it require too much strengthening of the larger wings or something like that?

For simplicity I put a number to it: 2 or 3 times bigger. Can fighter jets be made 2 or 3 times bigger and still have similar performance in maneuverability and thrust/weight ratio?

4 Answers
4

The question is not whether you can do it but why would you want to.

Fighters, like any other aircraft are designed around mission profiles; what do you want this aircraft to do, at what speeds, ranges, altitudes, combat radii, endurance, weapon carriage and employment, stealth, etc. Therefore you would need some sort of justification for building a fighter aircraft this size. That justification has to be so appealing that it can overcome the negative aspects of building a big fighter.

While these profiles vary considerably between individual aircraft, they are constantly curtailed by the brutal and impartial realities of war. Gen Stevens pointed out 4 fundamental requirements of a fighter in one of his critiques of the F-35. These are important to remember as they have been integral in every successful fighter design over the past century of air combat. Unfortunately, the larger the fighter, the more diametrically opposed to these realities they are and the greater the probability of failure in combat.

ATTACK YOUR ENEMY IN OVERWHELMING NUMBERS

This one is fairly simple to understand. The greater your force is, the more difficult it is for the enemy to destroy you. But defense ministries have limited budgets and can only spend so much on new fighters. So the choice becomes should I buy a few advanced and highly capable fighters or lots of cheap, single purpose aircraft. Air warfare has favored the latter. Not only are smaller aircraft cheaper to procure, they are cheaper to operate as they burn less fuel and require less maintenance. Consequently. Your nation's pilot pool can get a lot of flight time in them and can become highly proficient in their operation.

SPOT YOUR ENEMY BEFORE HE SPOTS YOU

Which is going to be easier to spot from a distance: a 747 or an F-5? Which is more likely to have a lower radar or IR signature if both employ LO features?

OUTMANEUVER AND OVERWHELM HIM

By the energy maneuvering equation, fighter design had always favored the following features:

High thrust to weight ratio

Low wing loading

Low specific fuel ratio

And this tends to favor a fighter which is basically a small, fast, nimble gas can with a compact but powerful engine and an effective armament for engaging and killing other fighters. Not easily achievable with a larger aircraft.

KILL HIM QUICK AND MOVE ON

So now, a large, expensive, easy to spot aircraft manned by low time flight crews is now forced to engage a swarm of nimble, fast turning fighters which not only can quickly turn inside him and get to an optimal firing solution faster and more reliably but are also more difficult to engage and shoot using BVR weapons now has a very limited probability of survival.

For those reasons, history does not favor a large fighter unless it is equipped with some kind of mission system giving it a tactical advantage - and even then that is a large obstacle to overcome in terms of cost, strength, and weight.

I liked the answers given here. I wanted to add 2 specific examples not mentioned yet, along with an analogy, to help add to what has been posted.

As mentioned, the design of a tactical military jet is a trade-off between effectiveness and ineffectiveness for a given mission. A simple example is that of a light attack bomber. Precision bombing would dictate a stable platform, i.e. low roll rates. The A7-E had a high mounted wing to increase stability in the bombing run.

As a bomber the A7-E was unsurpassed. I was in a bombing derby in 1985 with the newly deployed F-18's. The F-18 had an advanced delivery system, and still I was able to match their CEP bomb for bomb. But the A7-E it was not a fighter. It did not have the thrust-to-weight ratio that a fighter did, and so was at a disadvantage in ACM with an adversary like the F-18.

The A7-E also operated off of carriers based at sea, which can complicate the choice of design. Bombers want long legs to reach inland targets with heavy payloads of bombs, and so fuel efficiency is of particular importance. The A7-E delivered in this respect, and burned around 6-9K of fuel per hour at sea level and military power. This got us up to around 750 mph, which is just below Mach 1. But long legs are not only needed to reach inland targets from the ship, they also serve maintaining real-time surveillance of threats surrounding the carrier out to 200 miles.

When the A7-E was replaced by the F-18, which has the shorter legs of a fighter, the carrier lost the ability to have real-time surveillance of surface threats out to 200 miles. So here you can see the tradeoff affecting the strategic concerns of the battle group.

My last comment concerns design choices for the F-14, which was designed as a long-range interceptor. I knew some Tomcat drivers while stationed aboard the USS Nimitz and they told me an interesting vulnerability that you might not guess. The F-14 interceptor has a variable-swept wing, which is an attempt to mitigate the interceptor design with that of a fighter designed solely for air-to-air engagements. Yet when the Tomcat was in a turn, and G's applied, the wings swept forward making it a barn door in the sky. Easier to spot than let's say the F-16. There was a saying in air-to-air combat something like, "The first one to gain sight wins the fight." So we see here another choice where survivability is sacrificed for the interceptor role.

Finally, consider the evolution of the design of 12 meter class sailing boat. The design of the 12 meter class boat must obey the formula (America's Cup formula):

Where L is waterline length, d is the difference between skin girth and chain girth, S the sail area, and F the freeboard. Designers are free to choose these parameters any way the want as long as when substituted into the formula in comes in at less than 12 meters. Make a sailboat good in all wind conditions, and then in strong winds it will probably be beat by a boat built strictly to win there.

Why don't they make them any bigger to get more range (or endurance)?
Is there a practical limit? Would it require too much strengthening of
the larger wings or something like that?

The SR-71 proved (in a manner of speaking) that there are few practical limits in aircraft design if someone is willing to foot the bill. while that was a high altitude recon plane it still taught the world a lot about extreme aircraft design, so I would say sure what ever you are dreaming can be built.

Whats important is that there is no practical physical limit so to speak but at the same time there is simply no need for such a plane. The key thing to remember is that production planes are built for missions not for exploring physics and engineerings. The fighter role is a short range, nimble plane that carries a light to medium weapons load. Its easier to have a refuel plane loiter near buy and aid the fighter than build a larger, less nimble fighter that can fly farther on its own. The plane you describe might even look close to a B-1 if it were to be built. The fact remains that its more than possible its just not necessary.

$\begingroup$The SR-71, while a fast plane, was not a fighter, and it wasn't even very maneuverable. The design load factor limit was just 3.5g's, less than the Cessna 172's so many of us fly (3.8g's).$\endgroup$
– Ron BeyerSep 10 '16 at 3:59

$\begingroup$I kinda agree with your point in the 3rd paragraph, but the SR-71 is a bad example to compare to. It was designed for stealth and speed, not maneuverability. I would have a very bad feeling if it tried to pull 9 g's even at subsonic speeds.$\endgroup$
– DrZ214Sep 10 '16 at 4:24

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$\begingroup$@RonBeyer It was derived from YF-12, was it not? Foxbat MiG 25 was a fighter, but it wasn't built for a turning fight; it was built as a high speed, high altitude interceptor.$\endgroup$
– KorvinStarmastOct 3 '16 at 12:47

$\begingroup$I believe the SR-71 was used as an example of what can be done, not of a large fighter. All sorts of engineering challenges were overcome to build the plane because someone was willing to pay for it and there was a need for its design mission. Same can be said for a much larger fighter. if there's a need and a budget, it can be done.$\endgroup$
– FreeManMay 23 '17 at 12:00

I think practical answers are that large planes are typically not agile planes and present a larger target for enemy fire.

I'm assuming that this fighter has to perform at least as well as a current front line fighters so you're talking about slinging a lot of mass around at high speeds. That takes fuel, and fuel adds weight. And to push more weight through thr sky you need more fuel, which adds more weight...so more fuel. At some point a long range fighter becomes an attack plane that can also happen to shoot down other planes.

Max weight of an F16-50 is 46,000 lbs. 3x would be pushing 140,000 lbs, or just shy of the MGTOW of a 737-600. If you stencil "FIGHTER" on to the side of a Lufthansa plane you'll have a giant fighter! It's perfect...nobody will suspect it of being a deadly long range fighter until it's too late!! It doesn't need to be fast, it just has to rapidly dispense dozens and dozens air-to-air missiles and a cascade of chaff and flares. Eventually the defending air force will simply run out of fighters at which point the giant plane just turns around to the nearest airline hub where it quietly blends back in with the rest of the airplanes.

$\begingroup$It would play the same role as the Su-27, which is long range air superiority fighter. Last I checked the Su-27 had a range of around 3000 km, which is very very high for a fighter. The role of my idea would be a long range air superiority fighter with even longer range. I like your 3rd paragraph, tho, lol. Whole new meaning to stealth.$\endgroup$
– DrZ214Sep 10 '16 at 4:24

$\begingroup$Alternatively, the role could be just to have a longer endurance. All else being equal, the fighter with more fuel will outlast the enemy. If two fighters match each other in maneuverability, such that they can only chase each other's tails, the one who runs out of fuel first (or is forced to flee and show his tail) will lose.$\endgroup$
– DrZ214Sep 10 '16 at 4:39

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$\begingroup$I believe a more practical solution is to produce unmanned fighters able of sustained maneuvering beyond 9g. It's the meat sock in the cockpit that limits a fighter's performance, not the machine. This article (popsci.com/ai-pilot-beats-air-combat-expert-in-dogfight) describes the future of air combat. It's only a matter of time before the tech is perfected.$\endgroup$
– acpilotSep 10 '16 at 10:47

$\begingroup$Yes definitely an unmanned fighter can be less limited, but the question still remains as to how big we can build them. I guess another way of asking it is, can something like the 737 be built strong enough to pull 9 g's? Or do material limitations prevent that?$\endgroup$
– DrZ214Sep 10 '16 at 20:40